A requirement for increased capacity in the U.S. cellular radio system has resulted in adoption of digital technology. The digital system employs time division multiple access (TDMA) as a channel access method in conjunction with a digital modulation scheme. A proposed standard (IS-54) for digital cellular communication specifies a particular frame and slot structure. Under this standard, three to six users share a common 30 KHz radio frequency (RF) channel. Each user transmits data in an assigned time slot which is part of a larger frame. The gross bit rate of data transmitted over the channel is 48.6 Kbits/sec. The transmitted digital data is first mapped onto pi/4-shifted differentially encoded quadrature phase shift keying (DQPSK) symbols and then pulse shaped using a square root raised cosine filter. The pulse shaped signal is subsequently modulated onto an RF carrier.
Data transmission in this digital cellular system is adversely affected by multipath propagation which causes delay spread and consequent inter-symbol interference (ISI), where a symbol is comprised of a pair of binary bits. Delay spread is expressed in terms of a quantity called delay interval, which is measured as the time interval between the first ray and last significant ray arriving at the receiver. Delay spreads exceeding one third of the symbol duration cause a significant increase in bit error rate (BER), necessitating use of an equalizer in the receiver. Typical delay spreads encountered in urban and rural areas in the U.S. are less than 40 microseconds, implying a need for equalization of one symbol of interference (40 microseconds) for a data rate of 48.6 Kbits/sec. Mobile receivers also experience rapid channel variations and Doppler induced frequency shifts that are proportional to vehicle speed.
The channel impairments described above require that nonlinear adaptive equalizers be incorporated in cellular radios. Two suitable equalizers are the decision feedback equalizer (DFE) and an equalizer based on a maximum likelihood sequence estimator (MLSE). The MLSE method employs the well known Viterbi algorithm and is referred to in the art as a Viterbi equalizer or an MLSE-VA equalizer.
Both the MLSE and DFE techniques have been researched in some detail for the European CEPT/GSM cellular radio system. Results of this research are reported by, for example, R. D'Avella et al. "An Adaptive MLSE Receiver for TDMA Digital Mobile Radio", IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, pp. 122-129, Jan. 1989, G. D'Aria et al. "Design and Performance of Synchronization Techniques and Viterbi Adaptive Equalizers for Narrowband TDMA Mobile Radio", proceedings of 3rd Nordic Seminar on Digital Land Mobile Radio Comm., Copenhagen, Denmark, Sep. 13-15, 1988 and A. Baier et al., "Bit Synchronization and Timing Sensitivity in Adaptive Viterbi Equalizers for Narrowband TDMA Digital Mobile Radio Systems", proceedings of IEEE Vehicular Technology Conference, Philadelphia, pp. 377-384, 1988.
The CEPT/GSM system, however, is quite different from the system proposed for the U.S. in that it employs a narrower time slot, partial response modulation Gaussian Minimum Shift Keying (GMSK), a wider bandwidth (200 KHz) and a higher data rate (270.8 Kbits/sec.). The narrower time slot typically permits the channel to be treated as being time invariant while the wider bandwidth implies a reduced fade depth. The higher data rate results in increased ISI. As a result, the channel equalization requirements of the European and the proposed U.S. cellular systems are different.
A fractionally spaced DFE (FS-DFE) technique is disclosed by the present inventors S. Chennakeshu, A. Narasimhan and J. B. Anderson in copending and commonly assigned U.S. patent application Ser. No. 07/754.105, filed Sep. 3, 1991, entitled "Decision Feedback Equalization for Digital Cellular Radio". This DFE technique employs a complex fast Kalman algorithm to track channel variations. The fast Kalman algorithm described therein is an extension of a type taught by D. Falconer et al. in "Application of Fast Kalman Estimation to Adaptive Equalization", IEEE Trans. Comm., Vol. COM-26, No. 10, pp. 1439-1446, Oct. 1978. The extensions provide for the use of a complex form without matrix inversions and stabilization of the algorithm, for finite precision implementation, through the addition of appropriate dither signals.
An alternative technique employs an equalizer based on a lattice structure and is disclosed by the present inventors S. Chennakeshu, A. Narasimhan and J. B. Anderson in copending and commonly assigned U.S. patent application Ser. No. 07/753,579, filed Sep. 3, 1991, entitled "Order Recursive Lattice Decision Feedback Equalization For Digital Cellular Radio". This DFE method employs a least squares (LS) adaptive algorithm to achieve convergence and tracking properties similar to that of the complex fast Kalman algorithm.
Another technique achieves equalization through MLSE demodulation. There are two fundamental approaches to realizing a MLSE demodulator. One of these approaches is described by G. D. Forney in "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference", IEEE Trans. Info. Theory, Vol. IT18, pp. 363-378, May 1972. Forney's approach uses a Viterbi algorithm to realize the maximum likelihood sequence estimator, with a squared metric that is derived based on the assumption that the additive noise in the received signal, at the input of the maximum likelihood sequence estimator, is white and Guassian. This is accomplished through use of a whitening filter at the input of the maximum likelihood sequence estimator.
The other MLSE approach is described by Gottfried Ungerboeck in "Adaptive Maximum Likelihood Receiver for Carrier Modulated Data Transmission Systems", IEEE Trans. Comm., Vol. COM22, No. 5, pp. 624-636, May 1974. This MLSE approach employs a matched filter followed by a MLSE algorithm and an auxiliary channel estimation scheme.
The technique of Ungerboeck is currently favored, primarily to circumvent stability problems associated with use of the whitening filter. The Ungerboeck MLSE demodulator includes a transversal filter to approximate a matched filter, use of the Viterbi algorithm to implement the MLSE and a set of LMS or gradient algorithms to perform adaptation functions for the channel estimation. Differences in design of the matched filter and channel estimation algorithm account for range complexity and performance differences in various implementations of this demodulator. The equalization approach used in the CEPT/GSM system (as described by R. D'Avella et al. in "An Adaptive MLSE Receiver for TDMA Digital Mobile Radio", IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, pp. 122-129, January 1989) is similar to Ungerboeck's approach, except that the initial estimate of the channel impulse response is acquired through a correlation based search.
As was previously stated, however, the significant differences between the CEPT/GSM and the proposed U.S. digital cellular systems preclude the adaptation, without significant and non-trivial modification, of the CEPT/GSM equalization technique in the U.S.
It is thus an object of the invention to provide an adaptive MLSE-VA receiver for a digital cellular radio system that is suitable for use with the proposed U.S. cellular signalling standard.